The basic linear pocket feeder is a known device for introducing coarse dry granular solids into a vessel or reactor containing gas at a pressure of a few atmospheres or more. Such a device consists of a tube for conveying solids by means of a circulating chain of pistons fitting tightly in said tube, forming so-called pockets that move through the tube. Typically, solids are poured into a hopper over a slot in the horizontal tube. The pockets convey the solids to another slot or bottom opening in the tube connected to a pressurized receiver vessel, into which the solids fall. Examples of linear pocket feeders are illustrated in copending U.S. applications Ser. No. 166,151 now Pat. No. 4,379,670, and Ser. No. 254,779, now U.S. Pat. No. 4,410,429.
Such linear pocket feeders are often used to feed coal in coal conversion processes. Coal feeding is one of the critical steps common to all coal conversion processes such as gasification, liquification and fluidized bed conversion. Slurry pumps and lock hopper feed systems are the two systems currently available for feeding coat at high pressures. For those processes that do not require solvent or quench liquid, slurry systems impose a process thermodynamic penalty, and both lock hoppers and slurry pumps become expensive in construction and maintenance in large sizes and at high pressures. Advanced coal feeders which are capable of economically feeding dry coal continuously into the more advanced coal conversion processes are needed, particularly since the longer term trend with these processes seems to be to higher pressures and higher feed rates.